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178

IRON

(;j) 10 grains of pure sodium thiosulphate and 3 grams of ammonium
carbonate, are dissolved to i litre ; TO c,.c,. of the iodine* solution arc
decolorised by about. 15 c,c. of the thiosulphate solution.

(4) Starch paste.

'rilntlioii of the lliioxitlplnttc solution. The tliiosnlphate solution is run
from a burette, with continual shaking, into a. bottle like those shown at
MIC1 (Fig, i.j) containing .100 c,c. of /inc. acetate solution and a, measured
volume (e.g., 10 e.e.) of the iodine solution. When a pale yellow colour
is readied, the titralion is completed in presence of starch paste. If 10
c.c, of the iodine solution require, say, 15 c.c. of the thiosulphate, I c.c.
of the latter will be equivalent to o-oi ', 15 o-ooodd gram of sulphur.

.I/'/'(irttlitx :   The apparatus shown in Mg.  i,; is required, including:

Two Kipps, one lor carbon dioxide and the other for hydrogen.

A wash -hot tie ,1 containing about 150 c.c. of .>",', silver nitrate solution;
both gases are pmUied by passage through this solution.

Two wash bottles 11 and />'' conlainiiu; about, loo c.c. of tin1 2% silver
nitrate solution and distilled water respectively.

A llask (' of 500 c.c, capacity (the Corleis Mask used for determining the
total carbon does well) furnished with a three holed stopper, traversed by
(t) a. right angled tube reaching almost to the bottom of the vessel and
serving to conduct the hydrogen and carbon dioxide, (2) the stem of a
hipped-funnel for introducing the acid to attack the metal, and (j) the,end
of a, small condenser conneetrd with a porcelain lube glax.ed internally
which can be healed to cherry-redness in a, suitable furnace 1).

An absorption llask 1C containing ,;oo c.c. of the /due, acetate solution
and a second Jt1 containing 50 c.c, of the same solution and serving as check.

Procedure. The 11 asks H and 1C1 being charged, 5 grains of the metal are
placed in the llask and the air in the apparatus expelled by carbon dioxide,
Tin1 porcelain tube is then gradually heated to redness and a mixture of
do c.c, of dilute hydrochloric, acid (i vol. HO to 2 vols. of water) with 30
c.c. of dilute sulphuric acid (i vol. U...S()4 to 4 vols. of water) run in by
means of the, funnel. The acids are allowed to act for some time in the
cold ; in the. meantime, the current of carbon dioxide is replaced by one

y()2H giums;   i c.c. of tlif. solution containing  y'qa.S  grams tf iodine per litre will,
tlierelxu'o, correspond with o'ooi grain of sulphur.gen in a